What do the options mean as shown in the picture attached?
When I set the grid size 10, and if I want 10 meters long beam, I need to input 100. (Why is it?)
So if I use these units mass(kg), and time(s), what is the unit of force?

Those options in the grid editor are just parameters to set up the grid.
The grid a pure graphic item and has nothing to do with the unit system.
When you set the grid size = 10 (units) you are just telling STKO that the grid step for the grid-snap is 10 units.
But remember that in STKO we don't have a unit system, so it's up to you to tell what 10 units mean.
For example if you are modeling in millimeters, then with a grid size = 10 (units = millimeters) you will have a grid-snap every 10 millimters, and to draw a 10 meters long beam, you need to cover 100 grid samples.
But as I told you this has nothing to do with unit system